BAGHDAD — Two mentally retarded women strapped with remote-control explosives — and possibly used as unwitting suicide bombers — brought carnage yesterday to two pet bazaars, killing at least 73 people in the deadliest day since Washington flooded the capital with extra troops last spring.

The coordinated blasts — coming 20 minutes apart in different parts of the city — appeared to reinforce American claims that Al Qaeda in Iraq may be increasingly desperate and running short of able-bodied men willing or available for such missions.

But they also served as a reminder that Iraqi insurgents are constantly shifting their strategies in attempts to unravel recent security gains around the country. Women have been used in ever greater frequency in suicide attacks.

The twin attacks at the pet markets, however, could mark a disturbing use of unknowing agents of death.

Iraq’s chief military spokesman in Baghdad, Brigadier General Qassim al-Moussawi, said the women had Down syndrome and may not have known they were on suicide missions. He said the bombs were detonated by remote control.

In Washington, Secretary of State Rice said the bombings prove Al Qaeda is “the most brutal and bankrupt of movements” and will strengthen Iraqi resolve to reject terrorism.

The first bomb was detonated about 10:20 a.m. in the central al-Ghazl market, the home of a weekly pet bazaar with various small animals but mostly birds. At least 46 people were killed and more than 100 wounded, said police and hospital officials.

Police said the woman wearing the bomb sold cream in the mornings at the market and was known to locals as “the crazy lady.”

The pet bazaar has been bombed repeatedly, but with violence declining in the capital, the market had regained popularity as a shopping district and place to stroll on Fridays, the Muslim day of prayer.

But yesterday, it was returned to a scene straight out of the worst days of the conflict. Firefighters scooped up debris scattered among pools of blood, clothing, and pigeon carcasses.

A pigeon vendor said the market had been unusually crowded, with people taking advantage of a pleasantly crisp and clear winter day after a particularly harsh January.

“I have been going to the pet market with my friend every Friday, selling and buying pigeons,” Ali Ahmed, who was hit by shrapnel in his legs and chest, said. “It was nice weather today and the market was so crowded.”

He said he was worried about his friend, Zaki, who disappeared after the blast about 40 yards away.

“I just remember the horrible scene of the bodies of dead and wounded people mixed with the blood of animals and birds, then I found myself lying in a hospital bed,” Ali said.

About 20 minutes after the first attack, the second female suicide bomber was blown apart in a bird market in a predominantly Shiite area in southeastern Baghdad. As many as 27 people died and 67 were wounded, police and hospital officials said.

The 21-year-old owner of a cell phone store, Rae Muhsin, said he was walking toward the New Baghdad bird market when the explosion shattered the windows of nearby stores.

“I ran toward the bird market and saw charred pieces of flesh, small spots of blood, and several damaged cars,” Mr. Muhsin said. “I thought that we had achieved real security in Baghdad, but it turned that we were wrong.”

The bombings were the latest in a series that has frayed Iraqi confidence in the permanence of recent security gains.

The American military in Iraqi issued a statement that shared “the outrage of the Iraqi people, and we condemn the brutal enemy responsible for these attacks, which bear the hallmarks of being carried out by Al Qaeda in Iraq.”

The American ambassador to Iraq, Ryan Crocker, said the bombings showed that a resilient Al Qaeda has “found a different, deadly way” to try to destabilize Iraq.

“There is nothing they won’t do if they think it will work in creating carnage and the political fallout that comes from that,” he said in an interview at the State Department.

President Jalal Talabani of Iraq said the attacks were motivated by revenge and an attempt “to stop the march of history and of our people toward reconciliation.” He confirmed the death toll was about 70.

An American military spokesman, Commander Scott Rye, gave far lower casualty figures — seven killed and 23 wounded in the first bombing, and 20 killed and 30 wounded in the second.

He confirmed, however, that both attacks were carried out by women wearing explosives vests and said the attacks appeared coordinated and likely the work of Al Qaeda in Iraq.

Associated Press records show that since the start of the war at least 151 people have been killed in at least 17 attacks or attempted attacks by female suicide bombers, including yesteday’s bombings.

The most recent previous attack was January 16 when a female suicide bomber detonated her explosives among men preparing for the Ashoura holiday in a Shiite village in Diyala province, northeast of Baghdad.

While involving women in such deadly activity violates cultural taboos in Iraq, the American military has warned that Al Qaeda is recruiting women and young people as suicide attackers because militants are increasingly desperate to thwart stepped-up security measures.

Syria also has reportedly tightened its border with Iraq, a main transit point for incoming foreign bombers.

Women in Iraq often wear abayas, the black Islamic robe, and avoid thorough searches at checkpoints because men are not allowed to touch them and there are too few female police.

Even the use of the handicapped in suicide bombings is not unprecedented in Iraq. In January 2005, Iraq’s interior minister said insurgents used a disabled child in a suicide attack on election day. Police at the scene of the bombing said the child appeared to have Down syndrome.

Many teenage boys were among the casualties in the al-Ghazl bombing yesterday, according to the officials who gave the death toll. They spoke on condition of anonymity because they weren’t authorized to release the information.

In Late November, a bomb hidden in a box of small birds exploded at the al-Ghazl market, killing at least 15 people and wounding dozens. The American military blamed the November attack on Iranian-backed Shiite militants, saying they had hoped Al Qaeda in Iraq would be held responsible for the attack so Iraqis would turn to them for protection.

The American military has been unable to stop the suicide bombings despite a steep drop in violence in the past six months. Yesterday’s blasts were the deadliest in the capital since an April 18 suicide car bombing that killed 116 and wounded 145. Washington’s “surge” of an additional 30,000 soldiers into Baghdad and other parts of central Iraq began in February, but did not reach full strength until June.